Teton Dam Collapse: Engineering Failure, Mechanisms, and Legacy

Apr 21, 2026

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2 min read

YouTube video ID: J7ieKmP96Hc

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Teton Dam rose 305 feet (93 m) in southeastern Idaho and was finished in 1975. The Bureau of Reclamation built the dam for flood control, power generation, and irrigation, despite strong environmental and financial criticism that labeled the $100 million project as costly. To avoid losing a year of spring runoff, the Bureau began filling the reservoir before outlet works were complete. The original plan limited filling to one foot per day, but heavy spring melt prompted officials to relax that restriction.

Failure Mechanism

The dam employed a zoned embankment design with a loess core—wind‑deposited silt that is highly erodible. Its foundation rested on welded tuff from the Yellowstone Supervolcano, a rock mass that is fractured, porous, and prone to hydraulic fracturing. Pilot grout programs failed to seal the foundation, leading to the installation of core trenches and a triple‑row grout curtain. Water likely bypassed the dam through “windows” in the grout, poor compaction, or hydraulic fracturing, creating pathways for seepage. Because loess can maintain a roof and walls, the emerging tunnels—known as piping—did not collapse, allowing them to enlarge. The narrow, steep‑sided key trench produced an arching action that forced soil laterally into canyon walls, preventing the soil from collapsing into and sealing the developing pipes.

Collapse

Seepage first appeared on the right abutment on 3 June 1976. By 5 June at 7:00 a.m., water was exiting the dam face; at 10:30 a.m., a muddy geyser erupted. Bulldozers sent to plug the hole were swallowed by the embankment, and a whirlpool formed as the reservoir drained through the center. The dam breached around noon, five hours after the first major leaks were identified. The flood claimed 11 lives and devastated the towns of Wilford, Sugar City, Rexburg, Hibbard, and Salem.

Legacy and Lessons

Investigations concluded the disaster was not a freak accident but the result of prioritizing frugality over safety. The failure prompted the creation of standardized dam safety guidelines that now apply to all U.S. federal agencies. It also spurred research into filter and drainage design, as well as the mechanics of hydraulic fracturing in embankments. The episode reinforced the engineering culture’s need for rigorous geotechnical analysis, especially when building on fractured volcanic rock and using erodible soils.

Takeaways

The Teton Dam’s 1975 construction used a loess core on fractured welded tuff, creating a high risk of seepage and piping.
Relaxed filling limits and incomplete outlet works allowed water to bypass the grout curtain, triggering hydraulic fracturing and internal erosion.
Piping persisted because the strong loess maintained tunnel walls, while arching action in the key trench prevented natural sealing.
The five‑hour breach on 5 June 1976 killed 11 people and flooded multiple Idaho towns, illustrating the human cost of engineering shortcuts.
The disaster led to nationwide dam safety standards and renewed emphasis on geotechnical rigor in embankment design.

Frequently Asked Questions

What caused the piping failure in the Teton Dam?

Piping resulted from water seeping through poorly sealed grout windows and eroding the loess core, which remained strong enough to keep tunnel walls intact, allowing the tunnels to grow rather than collapse and seal the leaks.

How did the Teton Dam disaster change U.S. dam safety regulations?

The failure prompted the establishment of standardized dam safety guidelines across all federal agencies, mandating stricter geotechnical analysis, proper grout sealing, and robust filter and drainage designs to prevent similar collapses.

Does this page include the full transcript of the video?

Yes, the full transcript for this video is available on this page. Click 'Show transcript' in the sidebar to read it.

Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Geotechnical Engineering Soil Mechanics Textbook Recommended

Provides the foundational knowledge on soil behavior, piping, and embankment stability required to understand the Teton Dam failure.

Amazon →

Book On Famous Engineering Disasters

Offers historical context on structural failures and the evolution of safety standards in modern engineering.

Amazon →

Geology Of The Western United States Book

Explains the volcanic rock formations and geological conditions that contributed to the Teton Dam's foundation issues.

Amazon →

Pocket Soil Testing Field Kit

Allows for hands-on exploration of soil properties like erodibility and compaction, which are central to the Teton Dam's failure mechanism.

Amazon →

Links may be affiliate links. We only include resources that are genuinely relevant to the topic.

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Full Transcript YouTube

In 1975, after three long years 
of excavating, hauling, placing,  
and compacting soil across the Teton river, 
Teton Dam topped out at 305 feet or 93 meters  
high. Built by the Bureau of Reclamation, the 
dam was the flagship component of the Teton  
Basin project of southeastern Idaho, meant 
to provide flood control, power generation,  
recreation, and irrigation water supply 
for farmland in the Snake River Valley.
Not everyone was enthusiastic about the dam. The 
benefits were significant, but many felt that they  
didn’t make up for the environmental impacts, let 
alone the $100M price tag. Like most major water  
projects, it was controversial from the start. But 
politicians and dam advocates eventually won out.  
The lawsuits were resolved, and the earthen 
dam reached its final height at the end of  
1975. Other parts of the project were still 
under construction, even as the dam wrapped  
up. In particular, the contractor was behind on 
a tunnel through the left abutment of the dam,  
called the river outlet works. This structure was 
needed to make controlled releases downstream.  
By the time the dam itself was done, the smaller 
auxiliary outlet tunnel on the opposite side was  
the only way to release water. Even so, the 
Bureau was eager to put the dam in service.  
Waiting for the completion of the river outlet 
works would mean losing out on a whole year of  
valuable spring runoff that could be put to use 
irrigating fields, floating boats, supporting  
fish, and generating electricity. So, the Bureau 
decided to start filling the reservoir anyway.
The first filling of any reservoir is a risky 
process. No matter how much you plan a dam,  
you never know how it’s going to respond to 
the immense pressure of actual water. Like  
many projects of its kind, Teton’s designers 
required that the initial fill be moderated,  
only allowing the reservoir to come up by 
a maximum of 1 foot (or 30 cm) per day. As  
it filled, engineers and technicians performed 
daily inspections and monitored the water levels  
in nearby wells. The goal was a measured process, 
loading the structure slowly so they could catch  
any problems and respond, even lowering the 
reservoir back down if anything seemed off.
Early in 1976, though, after a particularly 
snowy winter, it became clear that the spring  
melt was going to send a lot of water their way. 
Without the river outlet works, the dam’s ability  
to release water was limited. The auxiliary outlet 
tunnel just couldn’t handle the flow they needed.  
That meant the reservoir would have to fill faster 
than the foot-per-day limit. So, they relaxed  
the limit. The dam seemed to be holding fine 
anyway, and there really wasn’t another choice.
As the summer got closer, the reservoir was nearly 
full, just a few feet shy of the spillway. On June  
3rd, engineers were doing their daily routines 
when they noticed water springing from the right  
abutment. The leaks were pretty far from the 
dam. It’s not completely out of the ordinary  
for little seeps to form in the vicinity as 
the local groundwater levels respond to a new  
reservoir. Two days later though, on the early 
morning of June 5, there were more leaks, and  
this time they were on the dam itself. Engineers 
quickly knew something was seriously wrong.
The water coming through the west side of the 
dam was first noticed around 7AM. By 10AM,  
the project’s construction engineer was 
peering into a tunnel carved through the  
embankment that was tall enough to 
stand in and extended into the dam  
as far as he could see. I’m Grady, 
and this is Practical Engineering.
Teton was a zoned embankment dam, meaning it 
was made of different kinds of compacted earth,  
each serving a different function. That soil came 
from borrow areas near the site of the project to  
minimize the cost of hauling it. Roughly 10 
million cubic yards or 7-and-a-half million  
cubic meters of material were excavated from 
the abutment areas and even from the bottom  
of the river, hauled to the site, and placed 
in thin layers, then compacted into place.  
In the center of the embankment was Zone 
1, built from wind-deposited silt (known  
as loess) that covered the upland areas at the 
top of the canyon around the dam. This zone of  
fine particles was the core of the dam, meant to 
provide the watertight barrier to prevent leakage.  
But even if the dam itself was watertight, 
there was another engineering challenge.
Below all the loess in the upland areas, 
the geology in southeastern Idaho gets a lot  
more complicated, thanks to an eruption of the 
Yellowstone Supervolcano about 2 million years ago.  
When a massive volcano erupts, it doesn't 
always flow like liquid basalt. Instead of  
oozing, it explodes into a gigantic cloud of 
hot, searing pulverized rock that’s somewhat  
misleadingly called volcanic ash. If that ash 
is hot enough when it lands, the weight of the  
layers above it squeezes the particles together 
into solid rock called welded tuff. But as that  
material cools, it also shrinks and cracks. 
Hot gases create voids. Soil and rocks picked  
up by the ash cloud create porous rubble 
zones. And seismic activity over millions  
of years breaks everything even further, 
leaving joints and fissures. As a result  
of all of those geologic processes, Teton’s 
foundation was essentially swiss cheese.
The Bureau of Reclamation carried out some 
early tests to try and seal up the porous  
rock with grout. Just a few holes to see 
how well it would work. They estimated it  
would take about 260,000 cubic feet of grout 
injected into the holes to seal them up. It  
ended up taking more than double that. Some 
of the holes simply didn’t stop taking grout,  
no matter how much they put in. The next year, 
they drilled holes to check if the grout worked  
out. The core samples they recovered were just as 
broken, fractured, and porous as before the pilot  
program. It just wasn’t going to work out. So the 
Bureau came up with a new plan: core trenches.
If they couldn’t seal up that worst top layer of 
foundation rock, they would just take the material  
out and replace it with something more watertight. 
So the designers of Teton incorporated key  
trenches below the dam, intended to cut off the 
flow of water underneath through the foundation.  
The trench was wider in the valley, but narrower 
up the abutments to help the dam “lock in” to the  
steep canyon walls. At the bottom of the trench, 
the Bureau went ahead with the grout curtain idea,  
but slightly modified from the pilot 
program. This time they drilled three  
rows of holes. The outer rows were grouted 
first to create an initial seal. Then grout  
was injected into the center row with the 
goal of completely filling the space without  
losing all the material upstream and downstream. 
Despite what they learned in the pilot program,  
the project required significantly more 
material than expected, ultimately injecting  
roughly 600,000 cubic feet (or 17,000 cubic 
meters) of grout into the dam’s foundation.
Once that was finished, crews backfilled 
the core trench along the bottom of the  
dam with the Zone 1 silt to create a 
watertight barrier. On the surface,  
it seems like a reasonable course of action. You 
have seemingly watertight materials from deep  
in the foundation all the way to the top of the 
dam, first with the grout, then the core trench,  
and finally the embankment on top. But some of the 
decisions made in that trench would prove fatal.
We’ll never know the exact 
single cause of the failure,  
since a lot of the evidence was washed away. But 
two teams of engineers investigated the wreckage,  
one from the Bureau of Reclamation 
and an independent panel. Both  
reached basically the same conclusion: the 
dam’s design was doomed to fail all along.
As the reservoir filled up, it reached the 
upper layers of rock that were the most  
fractured and started flowing into it. With 
no real barriers, the flow could move freely  
all the way to that silty core trench, and 
somehow, it could also get past it. It could  
have been that water flowed through “windows” 
in the imperfect grout curtain underneath.  
It could have travelled along an area that 
wasn’t well compacted during construction. Or  
the reservoir water pressure could have resulted 
in a phenomenon known as hydraulic fracturing,  
the same process used in oil and gas 
production, that you probably know as  
fracking. If water pressure exceeds the forces 
holding earthen material together (namely,  
the weight of the soil above), it can force 
open cracks, creating pathways for seepage.  
It’s possible all three contributed to the flow 
getting past the dam. All we know is that it  
found a path, and once it got past, it could flow 
freely in the cracks of the foundation downstream.
At 7:00 AM on June 3, 1976, 
two days before the collapse,  
that seepage was spotted on the right 
abutment. At the time, there was no way  
to know what path that water was taking. It 
was running clear, meaning it wasn't carrying  
away soil yet. But it was a hint of what the 
water was doing inside that fractured rock.
That Zone 1 material was well-compacted, 
but all it took was a small area of loose  
soil to break free. The issue is that, even when 
well-compacted, it’s not that hard for material to  
break away. Silt is a highly erodible material. 
Large particles like gravel and coarse sand  
mechanically lock together, plus they’re heavy, 
making it harder to pluck them free. Microscopic  
clay particles often stick together through 
intermolecular forces. But right in between, you  
have silt and fine sand. Too small to mechanically 
lock together, too large for intermolecular forces  
to dominate, and very lightweight. The loess 
soil from the plains along the river was  
practically the worst choice of material you 
could make for the core of an embankment dam.
Paradoxically, that erodibility was 
made worse by the silt’s strength.  
You’d think that a strong material would 
be a good thing in a dam, but in this case,  
you’d be wrong. Where other materials would 
naturally slump into voids as soon as they formed,  
“healing itself,” the silt was strong enough 
to maintain vertical walls and a roof. As it  
eroded away, it formed a tunnel, a process 
known as piping. Those pipes just created  
more space for water to flow and erode 
more of the dam away from the inside.
Another contributing factor was the narrow 
width and steep sides of the key trench.  
The soil distributed downward forces 
laterally into the rocky walls of the trench,  
kind of like an arch bridge converts loads 
into horizontal thrusts at the abutments. Had  
the full weight of the soil propagated straight 
down, it would have likely collapsed any tunnels  
forming through the core, but the arching 
action allowed them to stay open and grow.
Downstream, the rock was loose, and the 
fractures and voids were large enough to  
carry soil particles away. There was a "free 
exit" for the dam's internals. While Zone 2 of  
the embankment was intended to be a filter that 
would keep any erosion from carrying soil away,  
the actual seepage simply passed 
underneath it like it wasn’t even there.
By the morning of June 5, a channel 
had been hollowed out under the dam.  
Staff saw water exiting at the toe, and it was 
quickly getting worse. At 10:30 AM, a muddy geyser  
erupted from the face of the dam. Operators sent 
bulldozers to try and fill the hole, but the  
machines were swallowed by the eroding embankment, 
and the operators had to be rescued with ropes.
Only 30 minutes later, a sinkhole opened up on 
the upstream face, below the reservoir, giving the  
water a more direct path through the dam. Multiple 
witnesses saw a terrifying whirlpool form. The  
reservoir was draining like a bathtub directly 
through the center of the dam. More dozers  
tried in vain to push material on the upstream 
side to close the hole, but it was too late.  
The erosion worsened, allowing more flow, causing 
even more erosion in a runaway feedback loop until  
finally the embankment gave way at about noon, 
only 5 hours after the leaks were first noticed.
A wall of water thundered through the hole in 
the embankment. The breach wave moved quickly  
downstream in the Teton River, widening out once 
it left the canyon. The closest town in its path,  
Wilford, was basically wiped from 
the map, with nearly all the homes  
there swept away. Further downstream, Sugar 
City and Rexburg were similarly decimated,  
with the vast majority of the buildings 
rendered a complete loss. 
Then the smaller farming communities of Hibbard and Salem were inundated, homes and livelihoods washed away.  
The breach killed thousands of livestock, 
destroyed railroads, bridges, and farmland,  
and left thousands without a home. Ultimately, 11 
people lost their lives. Had the failure happened  
at night when no one was watching the dam, it 
could have been hundreds or even thousands more.
Investigations concluded this was not a "freak 
accident." The challenges of erodible soil and  
fractured geology were well known long before 
the 1970s. The Bureau simply didn't spend enough  
money to address them. In the end, it was a 
simple case of frugality over safety. As one  
investigation said: “Defensive measures, such 
as rock surface sealing and adequate filters,  
were well within the state-of-the-art at the time 
Teton Dam was designed and should have been used.”
But even though it wasn’t a novel failure 
mode, the impact Teton had on the engineering  
community was enormous. 
To see a brand 
new structure, built by one of the most  
technically competent agencies in the US, fail in 
such dramatic fashion was catalyzing in many ways.  
The federal government responded by 
standardizing dam safety guidelines  
across all federal agencies involved in dam 
construction, rules that are still in place  
today. The event also spurred research 
into filter and drainage design for dams,  
along with a better understanding of the 
mechanisms of hydraulic fracturing. Finally, it  
showed engineers everywhere what was possible when 
geotechnical issues aren’t taken seriously enough.
The very first dam I worked on in my career 
was very much “Teton-esque.” It was designed  
by another firm, but during construction, 
they realized that the foundation was much  
worse than expected. There were pores, 
fractures, and even caves below the dam.  
They had to shut down the job and re-engineer it 
entirely. Our solution was a deep cutoff wall.  
It basically involved running an enormous 
vertical chainsaw along the length of the dam,  
then filling the trench with a watertight mix 
of rock cuttings, cement, and bentonite clay.
I vividly remember working through the 
complexities and challenges of dealing  
with that foundation material. It’s really easy 
to understand these failure modes in hindsight.  
But when you’re actually working on a real 
project, it’s hard to visualize how soil,  
rock, and pressure will actually behave 
underground. Things get pretty abstract,  
and you can lose sight of the stakes. It’s 
hard to explain what it meant to have that  
example of Teton at the top of mind for everyone 
on the design team. For all the computer models,  
testing data, and equations, we also had 
a real life example of what could happen  
if we got it wrong. It was grounding in a way 
that textbooks and excel spreadsheets are not.  
And I like to think that’s how we honor 
the victims of engineering disasters: to  
share the stories and keep the lessons from being 
forgotten to make sure it doesn’t happen again.
The story of Teton Dam is in many ways a story about geology. I think it’s kind of human to just think that the ground is the  
ground. It does the same job no matter where you. 
The truth is that geology affects us in all kinds  
of interesting and weird ways. My fellow creator 
Joe Scott recently released a documentary where he  
visited the oldest and the newest places on earth. 
It’s this fascinating exploration of geology,  
how it affects us, and the wild shift in 
perspective you need to really wrap your head  
around the topic. The whole documentary is less a 
story about rocks and more about how they help us  
reframe our understanding of the earth. I love it, 
and I’d love for you to check it out on Nebula.
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